Various types of ignition systems are known in which an ignition coil stores electromagnetic energy, under control of a semiconductor switch, typically a power transistor. It is desirable that the current flow through the ignition coil and through the semiconductor switch be controlled in such a manner that, under all conditions of operation of the engine, that is, under varying speeds, the electromagnetic energy stored in the ignition coil is just sufficient to provide an effective spark at a spark plug. Permitting current flow to exceed this electromagnetic storage causes heating of the ignition coil and of the semiconductor switch and hence causes comparatively high losses in the ignition system while, additionally, possibly leading to overload of the components of the ignition system. In various types of such systems, which operate highly satisfactorily and efficiently, the ON and OFF conditions of the semiconductor switch are controlled by a threshold switch which, in turn, is controlled from the engine transducer, typically a signal generator operating similarly to an a-c generator. The threshold levels of the threshold switch are usually so set that they have a relatively small distance from the zero or null or cross-over value of the a-c signal provided by the signal generator. The threshold level must be close to the zero or null or cross-over level since, otherwise, at low engine speeds and when the peak signal level from the signal generator, itself, is low, failure of ignition signals might result. Yet, this low threshold level causes, at low-speed operation, unnecessarily long current flow through the ignition coil, and hence may cause excessive heating of the ignition coil and the switch connected thereto.